RFR: JDK-8302323 Add repeat methods to StringBuilder/StringBuffer [v8]

[Jim Laskey]

> Add the ability to repeatedly append char and CharSequence data to StringBuilder/StringBuffer.

Jim Laskey has updated the pull request incrementally with one additional commit since the last revision:

  Expand test for StringBuffer and illegal code points

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12728/files
  - new: https://git.openjdk.org/jdk/pull/12728/files/7d3ecc77..1a77988b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12728&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12728&range=06-07

  Stats: 568 lines in 3 files changed: 388 ins; 180 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12728.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12728/head:pull/12728

PR: https://git.openjdk.org/jdk/pull/12728